Torsional springs find use in a variety of mechanical applications. When such springs are used for improving Noise Vibration and Harshness characteristics of mechanical devices, some form of material base damping is generally desired. Elastomer springs are therefore preferred over metallic springs for constructing such devices because of their relatively low cost and weight, and the relatively higher coefficient of viscous damping present therein.
There are two categories of torsional vibration seen in most mechanical applications utilizing rotating shafts. The first is recognized as rigid-body vibration where every point in the entire shaft oscillates angularly with the same amplitude. Such rigid-body vibration is characterized by low frequency and large amplitude and is isolated from permeating the rest of the system by the introduction of a Low Stiffness Torsional Spring-Damper. This is known as torsional vibration isolation and the corresponding device is known as a Torsional Vibration Isolator. The prime objective of a Torsional. Vibration Isolator is to prevent vibration due to the rigid-body motion of the shaft to be transmitted to the rest of the machine.
The second torsional vibration category is recognized as a flexible-body vibration where every point in the entire shaft oscillates angularly with varying amplitudes thereby resulting in the flexing of the shaft. Such flexible-body vibration is characterized by higher frequency and small amplitude and is attenuated from the vibrating shaft by absorbing it with a torsional spring-inertial system. This is known as torsional vibration absorption and the corresponding device is known as a Torsional Vibration Absorber. The prime objective of the Torsional Vibration Absorber is to lower the amplitude of vibration at the shaft to acceptable limits thereby preventing its premature failure in fatigue.
The most common application for a Torsional Vibration Absorber is the crankshaft torsional vibration damper where the frequencies are high (usually between 150 Hz. to 350 Hz.) and the conventional construction methods work adequately. However, there are several other applications where Torsional Vibration Absorbers are commonly employed in mechanisms and required to attenuate low frequency vibrations. Examples of such applications are replete in vehicle drive-shafts, prop-shafts, and half-shafts. This is where a Low Stiffness Torsional Spring-Damper is generally introduced along with a tuned inertia ring.
When a Low Stiffness Torsional Spring-Damper is desired for either a Torsional Vibration Absorber or a Torsional Vibration Isolator application, an immediately apparent problem is that the Low Stiffness Torsional Spring-Damper is structurally and modally unstable in the non-torsional directions (e.g. in the axial, conical, or radial directions relative to the central axis of the rotating shaft).
Structural instability refers to the Low Stiffness Torsional Spring-Damper potentially buckling or failing in any non-torsional direction under externally applied loads (e.g. belt-load in a crankshaft isolator causing conical buckling of a Torsional. Vibration Isolator. Modal instability refers to the Low Stiffness Torsional Spring-Damper potentially buckling or failing in any non-torsional direction under internal loads (e.g. axial dynamic loads in a drive-shaft caused by the increased motion of the inertia ring during operation. This causes the Torsional. Vibration Absorber to move axially and dislocate from its installed position. Such Low Stiffness Torsional Spring-Dampers are constructed with large volumes of elastomer, and with elastomer to metal bonding to prevent a failure due to the aforementioned instabilities. These constructions make the Low Stiffness Torsional Spring-Damper an expensive mechanical component.
FIG. 1 illustrates a conventional Torsional Vibration Isolator widely employed in automotive applications—particularly in vehicle crank-shafts. The Torsional Vibrational Isolator most often comprises of a metallic hub 10 that is mounted on its cylindrical Inner Diametric surface most proximate the Axial Center-Line of the Torsional Vibration Isolator to the vibrating shaft via a press-fit. A metallic pulley 50 is held concentric to hub 10 by the Low Stillness Torsional Spring-Damper and a tubular bushing 60. The Low Stiffness Torsional Spring-Damper includes a tubular inner metallic sleeve 20 that is press fitted onto hub 10, a tubular outer metallic sleeve 40 that is press fitted into pulley 50, and elastomer 30 that is injection molded and bonded (mold-bonded) between sleeves 20 and 40. Tubular bushing 60 is press fitted onto the pulley 50 on its non-slippery Inner Diametric Surface ID and slip fitted onto the hub 10 on its slippery cylindrical Outer Diametric surface. This essentially allows pulley 50 to interface with hub 10 through the Low Stiffness Torsional Spring-Damper.
It must be appreciated here that mold-bonding of elastomer 30 to sleeves 20 and 40 is an expensive manufacturing operation. This process involves sandblasting sleeves 20 and 40, properly masking the unbonded surfaces, spraying the bonded surfaces with a primer followed by an adhesive, and finally loading sleeves 20 and 40 into an injection (or transfer) molding machine where elastomer 30 is fed into the allocated space. In addition, careful monitoring of the mold parameters (such as time and temperature) is imperative to ensure a proper bonding of the adhesive simultaneously with proper cure of the elastomer 30.
Furthermore, to relive post-mold tension (due to substantially different coefficients of thermal expansion of elastomer 30 verses sleeves 20 and 40) in elastomer 30, sleeves 20 and 40 must be press-fitted into hub 10 and pulley 50 respectively, thereby requiring tight dimensional tolerances and a complex assembly process. Furthermore, a large volume of elastomer 30 is necessitated to provide the adequate fatigue life and the requisite low stiffness required for vibration isolation. These factors make the Torsional Vibration Isolator an expensive product to manufacture.
FIG. 2 illustrates a conventional low frequency Torsional Vibration Absorber that is widely employed in automotive driveline applications—particularly in vehicle drive-shafts, prop-shafts and half-shafts. The Torsional Vibration Absorber most often comprises of a metallic hub 10a that is mounted on its Inner Diametric Surface most proximate the Axial Center Line of the Torsional Vibration Absorber the vibrating shaft via a press fit. A metallic ring 50a with a measured amount of Polar Mass Moment of Inertia is held concentric to hub 10a by the Low Stiffness Torsional Spring-Damper. The Low Stiffness Torsional Spring-Damper includes a tubular inner metallic sleeve 20a that is press fitted onto hub 10a, and elastomer 30a that is mold-bonded between sleeve 20a and ring 50a. This essentially allows ring 50a to interface with hub 10a through the Low Stiffness Torsional Spring-Damper.
As in the case of a conventional Torsional Vibration Isolator, it must be appreciated in the case of a conventional Torsional Vibration Absorber mold-bonding elastomer 30a to sleeve 20a and ring 50a is an expensive manufacturing operation (for the same reasons stated above). Furthermore, to relive post-mold tension (due to substantially different coefficients of thermal expansion of elastomer 30a verses sleeve 20a and ring 50a) in elastomer 30a, sleeve 20a must be press-fitted into hub 10a, thereby requiring tight dimensional tolerances and a complex assembly process. Furthermore, a large volume of elastomer 30a is necessitated to provide the adequate fatigue life and the requisite low frequency required for vibration absorption. These factors make the low frequency Torsional Vibration Absorber an expensive product to manufacture.
An additional concern in a low frequency Torsional Vibration Absorber is the structural and modal instability inherent in the device. Structural instability is due to the geometry of elastomer 30a (with through windows included for low stiffness) in addition to the type of elastomer compound utilized for such construction. Generally, elastomers with low carbon black particle loading are used for low frequency Torsional Vibration Absorber applications that give cause for potential fatigue failure (lowering carbon black particle loading lowers the dynamic shear modulus of the elastomer, but also weakens it being the binding substrate for the elastomer).
Modal instability originates due to the relatively low stiffness of elastomer 30a coupled with the mass and Polar Mass Moment of Inertia inherent to ring 50a, consequently allowing low frequency non-torsional vibration modes. It is common for the first mode-shape of a low frequency Torsional Vibration Absorber to be non-torsional in character, and for the device to have inadequate decoupling between the vibratory mode-shapes. Both these characteristics fall below industrial design best practices and are undesirable.
Another interesting point concerning conventionally constructed Torsional Vibration Absorbers is the elastomer 30a is the weakest structural link, and a failure thereof is catastrophic. Meaning that the entire sub-assembly disintegrates if the elastomer element fails. This is a serious safety concern as ring 50a that has substantial Polar Mass Moment of Inertia can become air-borne and cause damage to life and property.
Finally, a commonly realized problem common to both Torsional Vibration Isolators and Torsional Vibration. Absorbers is that they are not serviceable in the field. In fact, if the elastomer 30 (in a Torsional Vibration Isolator or 30a (in a Torsional. Vibration Absorber fails, most often the entire device is discarded, and a brand-new part is used as a replacement. At the very least, the device must be dismounted from the rotating shaft (often requiring the disassembly of the shaft itself from the machine), reworked (e.g. a new Low Stiffness Torsional Spring-Damper replaces the failed Low Stiffness Torsional Spring-Damper, re-balanced, and re-mounted back on the rotating shaft.
A novel design for a Low Stiffness Torsional Spring-Damper is therefore desired for Torsional Vibration Isolators and low frequency Torsional Vibration Absorbers, where despite having a low torsional spring stiffness, the Low Stiffness Torsional Spring-Damper has the following characteristics: (1) the elastomer is not mold-bonded to the two metallic extremities; (2) the amount of elastomer is relatively smaller compared to conventionally constructed devices; (3) the device only allows the torsional direction to be spring loaded; (4) there is some damping inherently present in the system; (5) the device does not disintegrate on failure of the elastomeric element; and (6) the device is serviceable in the field without disengaging from the rotating shaft.